Recently, as a next-generation NVRAM (Nonvolatile Random Access Memory) capable of operating at high speed to replace a flash memory, various kinds of device structures such as a FeRAM (Ferroelectric RAM), a MRAM (Magnetic RAM), an OUM (Ovonic Unified Memory) are proposed and a fierce development race is carried on to implement high performance, high reliability, low cost, and process compatibility. However, each above memory device has good and bad points and it is far from an ideal “universal memory” having all good points of a SRAM, a DRAM and a flash memory.
Based on the existing technique, a RRAM (Resistive Random Access Memory) comprising a variable resistor element in which an electric resistance is reversibly changed when a voltage pulse is applied has been proposed. The variable resistor element has an extremely simple structure in which a lower electrode 3 serving as a second electrode, a variable resistor 2 and an upper electrode 1 serving as a first electrode are sequentially laminated as shown in FIG. 1, and a resistance value can be reversibly changed when a voltage pulse is applied between the upper electrode 1 and the lower electrode 3. Anew nonvolatile memory device can be implemented by reading a resistance value in the reversible resistance changing operation (referred to as the “switching operation” occasionally hereinafter).
As a material of the variable resistor 2, a method of changing an electric resistance reversibly by applying a voltage pulse to a perovskite material known for its colossal magnetoresistance effect is disclosed in the following patent document 1 and non-patent document 1 by Shangquing Liu and Alex Ignatiev et at in U.S. Houston University. This is an extremely epoch-making method in which while the perovskite material known for its colossal magnetoresistance effect is used, a resistance change over several digits can be provided at room temperature without applying a magnetic field. In addition, according to an element structure disclosed in the patent document 1, a crystalline praseodymium calcium manganese oxide Pr1-xCaxMnO3 (PCMO) film that is a perovskite-type oxide is used as a material of the variable resistor 2.
In addition, it is reported that when an oxide of transition metal element such as titanium oxide (TiO2) film, a nickel oxide (NiO) film, a zinc oxide (ZnO) film, or a niobium oxide (Nb2O5) film is used as the material of the variable resistor 2, it shows a reversible resistance change according to a non-patent document 2 and a patent document 2. A phenomenon of the switching operation using NiO is described in detail in a non-patent document 3.
Patent document 1 U.S. Pat. No. 6,204,139
Non-patent document 1: Liu, S. Q. et al., “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films”, Applied Physics Letter, Vol. 76, pp. 2749-2751, 2000
Non-patent document 2: H. Pagnia et al., “Bistable Switching in Electroformed Metal-Insulator-Metal Devices”, Phys. Stat. Sol. (a), vol. 108, pp. 11-65, 1988
Patent Document 2: Japanese National Publication of PCT Application No. 2002-537627
Non-patent document 3: Baek, I. G. et al., “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses”, IEDM 04, pp. 587-590, 2004